Electric heating devices



2 Sheets-Sheet 1 Filed Au 7, 1967 Ira c572 i0 Pal/Z 5415261 Filed Aug.7, 1967 g 4, 70 P. EISLER 3,522,415

ELECTRIC HEATING DEVICES 2 Sheets-Sheet 2 3,522,415 ELECTRIC HEATINGDEVICES Paul Eisler, 57 Exeter Road, London, NW. 2, England Filed Aug.7, 1967, Ser. No. 658,785 Claims priority, application Great Britain,Aug. 8, 1966, 5,416/ 66 Int. Cl. H05b 3/36 US. Cl. 219528 16 ClaimsABSTRACT OF THE DISCLOSURE An electric heating device comprises aplurality of limbs, generally in zig-zag disposition, of flat sectionsheathed electric conductor, the width of the limbs being substantiallycoplanar and the limbs being held under longitudinal tension. The apicesof the zig-zag may be supported by smooth good insulators rotatable ontwo supporting rods one fixed and the other pulled by tension springs,so that all limbs are under substantially the same tension. Insulatingstrips may be interwoven to produce a loose fabric and such fabric maybe stored in the roll from which pieces can be cut. The sheaths need notbe impervious or of adequate insulating quality in themselves andmoisture may be driven off by preliminary operation at sub-normalvoltage. Various uses and applications are described.

This invention relates to electric heating devices and an object is toprovide a structure which can readily be adapted to various applicationsand loading and to provide a material which can be stored in longlengths from which a piece or pieces can be cut for the production of aheating device for particular cases.

According to the invention an electric heating device United StatesPatent 0 comprises a plurality of flexible limbs each forming a heatingelement and each being constituted by a generally flat section, sheathedelectric conductor, the limbs being spaced and disposed with their flatsurfaces substantially parallel so that when energised all radiate heatin the same general direction, and means exerting longitudinal tensionsolely on the sheaths of the limbs whereby the limbs are held suspendedin the said disposition. In particular a plurality of adjacent limbs ofthe device may be continuous and disposed in a zig-zag formation, thetension exerting means supporting the zigzag at the apices thereof withnegiligible friction. Then the tension is carried over from one limb tothe next and they will all be under substantially the same longitudinaltension. Briefly the supporting means may be smooth freely rotatablerollers or the like disposed within the apices.

In the simplest case the flat section of the limbs will define a more orless plane area (ignoring inevitable slight sag) from which heat will beradiated but if the limbs are spaced in the direction of their widthsthe atmosphere in which the device is used can flow over and betweenthem and be heated by convection. This spacing may differ in differentparts of the area and it may also be adjustable to enable the ratio ofheat dissipated by radiation to heat dissipated by convection to bevaried.

In the case of a zig-zag disposition, smooth rotatable rollers or thelike may be disposed in the two sets of apices those in each set beingcarried by a respective support. One support may be fixed in an externalskeleton framework and the other be secured in the framework by tensionsprings at intervals between the individual apices, serving to draw thissupport away from the other and so to impose tension on the sheath; orboth supports may be secured by tension springs.

Flexible strips, to which longitudinal tension is applied may beinterwoven with the limbs. Also a loosely woven "Ice framework to whichthe limbs are suspended, may be insuflicient of themselves to effectadequate electrical insulation of the conductors under all conditions ofoperation to be expected. For example if the sheaths are porous, underhumid conditions upon switching on the moisture might permit leakageunder the normal working voltage, but the moisture can be driven off bystarting at low voltage, as will be further explained. A particularlysuitable sheath is a single layer of glass fibre fabric.

A cover may be used on one side of the area occupied by at least some ofthe limbs, disposed to reflect back radiated heat so that the greaterpart of the total radiation will be in a direction away from the cover.The spacing of the cover from the limbs can be chosen according to thedesired convective flow around the limbs.

Again at least two sets of limbs can be superposed spaced apart, inorder to increase the total emission over a given area withoutincreasing the load/ area factor of the individual limbs.

The invention is not limited to such conductors or to any particulartype of conductors provided the sheathed whole is a flat structure. Theconstructions shown in my US. Pats. Nos. 2,971,073, 3,020,378,3,033,970, 3,149.- 406, 3,317,657 and 3,283,284 can all be used andother possibilities are carbon or graphite coated textile tapes, andcarbon layers between overlapping pieces of metal foil as in FIG. 11 of3,033,970.

The production of electric surface heating devices having relativelylarge areas emitting heat from narrow elements has hitherto usually beeneffected either by producing sheet materials with heating elementslaminated in them or stuck, pressed or otherwise fixed to them, or byinterweaving heating wires with insulating strands. The presentinvention on the other hand makes use of surface heating elements whichinter alia permits the same emission at lower surface temperature.

In many applications such as drying, baking or curing ovens and tunnelsrun at mains voltage or other voltage subject to safety regulations theheating elements used have hitherto either been provided with aninsulating sheath as prescribed by the regulations or been hermeticallyshielded by metal cladding or the like. Air or vapour has been guided topass over these heating elements by suitable trunking, baflles, blowers,etc. In view of the high temperature of the heating elements theinsulation and other features of the construction have been expensive,and the cyclic expansion and contraction of the heating element has ledto difficulties and failure.

The present invention avoids the difiiculties of expansion andcontraction, and since the limbs are suspended, usually in air, thesheath need not be of high insulating quality provided the suspendingmeans introduces adequate insulation at the points of suspension.

The conductors used in the present invention can be enclosed in aninexpensive, preferably fibrous sheath, e.g. non-impermeable woven orfelted glass fibre tapes. These sheath materials in the thin thicknessesused would not constitute an adequate insulation according toregulations, that is they would not maintain the required highresistance to earth of the heating conductor when subject to highhumidity, although they show this high resistance when dry, and theyprevent actual contact between the heating conductor and any solidobject. A short circuit however would have to be expected on switchingon if water condensed on the cold tape and an earthed metallic bodytouched the tape. 1

The present invention avoids this danger and also permits the use ofheating elements with meander-type conductive strands whereinneighbouring strands are at different potential and insulated from eachother substantially only by being spaced from each other by theirfibrous support or sheath, such as where they are sewn on to or intononimpregnated glass fibre tapes.

The invention will be further described with reference to theaccompanying diagrammatic drawings in which:

FIG. 1 is a perspective view of part of a heating device according tothe present invention,

FIG. 2 is a plan view of part of the device of FIG. 1,

FIG. 3 is a detail end view of FIG. 2,

FIG. 4 illustrates a roll of fabric in accordance with the invention,and

FIGS. 5 and 6 are details of two forms of sheathed conductor which canbe used in the invention, part broken away; and FIG. 7 is a diagrammaticview.

Referring first to FIGS. 1 and 2 a framework generally indicated at 11serves to support the limbs 12 of sheathed conductor. These limbs arecontinuous and form a zigzag the apices 13 of which are supported bybeing passed over glass or ceramic bobbins or tubes 14 constitutingsmooth surfaced cylindrical insulators of high insulating value. Theyare rotatably on supports formed by rods 15, 16, suitably of steel. Therod 15 is fixed in the framework 11 (which can be built for example ofslotted angle steel) by the aid of brackets 17. The rod 16 is drawn awayfrom the rod 15 by tension springs 18 anchored at suitable intervals tothe framework at a spacing which bring them clear of the limbs 12. Theweight acting on the rod 16 is carried by horizontal support brackets19.

The springs 18 impose longitudinal tension on the sheathed conductors,and since the tubes or bobbins 14 are smooth and can rotate withnegligible friction on the rods 15, 16 the tension is carried over fromone limb to the next by sliding of the sheathed conductors and/ orrotation of the tubes or bobbins and the whole zig-zag is undersubstantially the same tension throughout. Also this mode of suspensionapplies the tension exclusively to the sheaths. If the construction ofthe limbs is suitable (as will be described) no substantial tension willbe transmitted to the conductors themselves.

There may be a single tube or bobbin 14 at each apex, or the tubes orbobbins or some of them may be of such length that several apices aresupported at each. In the former case rotation alone is sufiicient toensure transmission of substantially constant tension from one limb tothe next. In the latter case some slip of some of the apices on the tubeor bobbin may also be involved. The tubes or bobbins may be slid alongthe rods as may be necessary in adjusting the spacing of the limbs. Someform of adjustable stop (not shown) e.g. tubular ferrules with grubscrews, may be fitted to the rods between the tubes or bobbins toprevent accidental axial movement.

The rod 15 could also be mounted by the aid of springs and supportingbrackets but this is usually unnecessary.

With the zig-zag so laid out, the width of each limb is substantiallyparallel to the general area of the zig-zag and when the conductors areenergised heat will be radiated generally in directions perpendicular tothe general area. Also with the apices 13 well spaced there is ampleroom for convective flow of air round the limbs.

If radiation mainly in one direction from the general area of thezig-zag is wanted, e.g. in the case of a drying tunnel, a cover may beused on the opposite side of the zig-zag e.g. constituted by the tunnelwall which reflects the radiated heat back. The spacing of the coverfrom the zig-zag controls convective flow and may be adjustable. Part ofsuch a cover is indicated in FIG. 1 at 21.

If increased heat dissipation over a given area is wanted withoutincreasing the load/area factor of the sheathed conductor and withoutbringing the limbs closer together, a plurality of zig-zags may besuperposed, sufficiently spaced to permit the desired convective flow.By way of example FIG. 1 shows two superposed zig-zags. The secondzig-zag has the parts marked with the same numerals as used in thepreceding general description.

The second zig-zag may equally be regarded as forming a second heater ofan oven or drying tunnel, the material to be heated constituting orbeing carried by a web which travels between the two zig-zags so that itis irradiated from opposite sides. Such a web may travel in severalsuccessive passes in opposite directions between a series of superposedzig-zags or pairs of zig-zags carried in a single framework, thusshortening the overall length of the oven or tunnel. If there is a pairof zig-zags per pass there may be dividing walls between the pairs,constituting covers having the action above described, though usuallythey will not be used because free vertical air fiow is desirable.

To assist in maintaining the desired disposition of the limbs of thezig-zag and reduce sag, well-spaced tapes 22 may be interwoven with thelimbs of the zig-zag and held under longitudinal tension e.g. byanchoring one end of each to the framework 11 and attaching the otherend by a tension spring not shown. These tapes may be of similarmaterial to the sheaths of the limbs.

These tapes 22 convert the zig-zag into a species of loosely wovenfabric. As shown in FIG. 4 such prepared fabric comprising sheathedconductors 12a. as the weft and tapes 22a as the warps can be stored inthe roll. Heaters may then be prepared by cutting off a length of thefabric and making connection to the ends of the conductors, or to theends and to intermediate points at which the conductors are if necessarysevered, to enable the conductors to be connected in parallel groups orother arrangements as may be necessary for the particular case.

FIGS. 5 and 6 show by way of example two possible constructions ofsheathed conductor which can be used.

In FIG. 5 there is a single wide conductor strip 25 e.g. of crimpedmetal foil such as steel, aluminium or alloy foil, the crimps 26 runningtransverse to the length of the strip 25. The width and thickness of thestrip will be chosen according to the required operating conditions andthe material of the strip which may be of other material than metal asmentioned earlier herein.

The strip 25 is enclosed in a two layer sheath, consisting of an innercovering 27 and an outer covering 28. Preferably the strip 25 is loosewithin the covering 27, and the latter loose Within the covering 28;then, in conjunction with the elasticity conferred by the crimping ofthe strip, no substantial tension will be transmitted to the conductorstrip while if the conductor strip should break, the elasticityconferred by crimping will give it a tendency for the two ends toretract at the break and convert this into a gap.

The coverings 27, 28 can be of plastics material or textile fabric, e.g.fibre glass fabric or one of one material and the other of the othermaterial. Plastics materials can be extruded or applied as separatedstrips and joined e.g. welded, struck, or sewn along the edge; or asingle strip may be folded over the conductor strip and the margins besimply overlapped or sewn welded or stuck together. Similarly textilefabrics can be woven or knitted in tubular form around the conductorstrip or be produced as tapes which are sewn along the edges, or asingle tape may be folded over with the margins overlapped or sewn orstuck together. Textile fabrics can be impregnated with plasticsmaterials.

In FIG. 6 a plurality of conductive strip 29 are used with transversecrimps 31. The sheath again comprises a loose inner covering 32 and aloose outer covering 33. The top and bottom of the inner covering 32 arejoined together between the strips 29 as indicated by dotted lines 34.These lines may represent stitching, or in the case of plastics,welding, or they may represent lines of adhesive. The effect is toprevent contact between adjacent strips 29 enabling them to be atdifferent potentials as may be necessitated by the scheme of electricalconnection necessary for the desired heating effect; the severalconductor strips in one limb or series of limbs may for example beconnected to provide a meander path (or several meander paths inparallel).

The outer covering 33 is here assumed to be of woven textile film butall the variations in material above described with reference to thecoverings 27, 28 of FIG. 5 are equally applicable to the construction ofFIG. 6. Also in either case one covering may be omitted so that thesheath is a single layer covering as in the case of FIG. 6, it is thecovering 32 which would be omitted since it is usually necessary to keepthe individual strips 29 separated.

The choice of materials above described with reference to the coveringsof FIGS. 5 and 6 also apply to the sheaths of FIGS. 1 to 3 and 4. Againinstead of tapes 22, 22a. cords could also be used, for example glassfibre cords.

The above description with reference to the drawings by no meansexhausts the possibilities of the invention and some further detailswill now be given.

As regards the tubes or bobbins of FIGS. 1 to 3 these may be mounted onmetal (steel) tubes which are rotatable on the rods 15, 1-6. Such metaltubes serve as bushings. Equally the rods 15, 16 may themselves betubular. The tension of the springs 18 may be adjustable e.g. by screwsand nuts. Usually bobbins will be used where only a single apex passesover the bobbin, the flanges axially locating the sheathed conductorswhile single tubes will be used where several apices are supported bythe one tube.

In the case of a fabric such as that of FIG. 4 the edges of the fabricare made especially strong, for example by using stouter tapes or cordsfor the warps at the edges. The edges can then be fixed to helicalsprings which are provided with a glass fibre sleeve over their ends orsome other gripping device which comes between the sheathed conductorand the metal of the framework and constitutes an electrical insulatorwhich is reliable electrically under all the conditions includingtemperature, humidity and liability to chemical attack prevailing at theposition of use Such a fabric having glass fibre insulation and glassfibre warps (whether produced when installed as in FIGS. 1 to 3 orpremanufactured as in FIG. 4) is capable of running at high temperaturewhile the spacing of the successive weft limbs determines the averagemaximum temperature or heat dissipation per unit area of the fabric.

By using quartz fibres still higher temperatures are available while byusing plastic film or fibrous nylon or cotton sheaths and warps acheaper fabric can be produced if the temperatures required are lowenough to permit.

When the heating fabric has to be decorative as well, the warps and/orweft can be coloured, printed, crimped or embossed. For low wattage persquare foot requirements the sheaths and warps can be extruded P.V.C.,and with such constructions the insulation of the tensioning and holdingprovisions operates essentially as a safeguard in case of softening ofthe plastics material along the edges of the fabric where the sheathedconductor is folded and over a small area is in effect under double wattloading.

The weakness of the insulation of the sheathed conductor used for thefabric of the invention is in most cascsparticularly in the case oftextile sheaths, comprising fabrics of glass and quartz fibres as wellas those of nylon, terylene or cottonthe breakdown of their insulatingvalue when water condenses on them or when they are needed to operate ina vapour or in highly ionised atmospheres. The sheathed conductors maybe designed so that they can be connected when starting from cold tosuch a low voltage that they get sufficiently hot while still on thatlow voltage to evaporate the water condensed on them. Thus they may berun, if in free air, just over C. in a low voltage circuit and switchedover to their normal running conditions after the water, if present,must have evaporated or boiled off.

If the heating fabric has to operate in a space with an atmosphere ofhigh inflammability or in a space where vapour threatens the breakdownof a permeable unim pregnated textile insulation the heating fabric maybe encased in a flat thin perforated box or bag of metal or othermaterial, and air or inert gas be pumped into the box with at leastslightly higher pressure than exists in the space filled withinflammable or highly ionised vapour or gas. Thus the chance of thesevapours or gases reaching the tape is eliminated and the need for usualexpensive precautionary measures for electric heating in spaces whereinflammable vapours are present is avoided.

The features described so far, particularly the permeability of theflexible fabric to air, the ease of adjustment or assembly in a framefor any desired shape and Watt/ sq. ft. loading, the safety provisionsarranged for, the low cost and robust nature of the fabric make itsuitable for use as a large area heater not only in ovens and dryingtunnels, but generally as a mobile, readily erected heater anywhere andfor open air heating of either goods, human beings or animals.

It can be used lying on the floor or ground or supported off the groundfor drying wheat, corn, hay and grain piled on it or as a blanket putover heaps of any agricultural products requiring drying while air canpercolate through them and water can flow off. It can be used suspendedunder the roof in passages and stables, as an acoustic ceilingsupporting a layer of sound deadening glass Wool, as a retractablecanopy over a terrace, pavement, outdoor seating arrangement (e.g. of astadium or coffee house, if desired underneath a usual canopy whichwould protect it from rain and reflect heat), as a curtain fabric forhanging on wet walls for drying purposes or over scaffolding on buildingsites for frost protection, as a blanket cover for accelerated curing ofconcrete, as a temporary cover over roofing or other surfaces to warmthem prior to pouring viscous compounds and thereby ease their flow, forpre-warming or thawing frozen equipment or engines, etc.

The mobile, ready for use nature of the fabric can be used forimprovising many types of surface heaters just when needed. The samefabric, fed preferably from a variable transformer, can be used as adrum heater for instance to melt solid wax to enable it to be poured outof the drum, it can be hung over a metal sheet to dry and cure wet paintand be wrapped around a build-up of resin impregnated laminated sheetsclamped in a mould for a boat hull, the curing of this reinforcedplastic structure being accelerated by the heat from the fabric. Thesame fabric can be laid under propagation boxes in the greenhouse orspread out in the garden and wet laundry placed on it, or perform manyother heating tasks, even indoors or in camping.

Being a flexible strong woven fabric it can be used as a heatablesupport for bedding or seats and when made with narrow components indecorative colours as a furnishing and curtaining fabric and it can evenbe used for sleeping blankets and heatable coats.

The industrial application of the fabric is not restricted to ovens anddrying tunnels where the frames holding the fabric can be positioned andmoved readily to suit any desired drying path or object and permitapplication of uniform radiant heat as well as convection of heat by airflow through the fabric. It can perform almost all conventional heatingtasks where heating of a large surface is desirable by a flexibleuniform low thermal inertia heater which can readily follow the surfaceto be heated in close proximity and can be brought to this surface whenrequired, necessitating no fixed installation. A particular applicationof the heating fabric is its use as conveyor belting essentially for thesimultaneous transport and heating of flat goods lying on it. Theheating and cooling of flat objects under pressure while they movecontinuously is an example of this application. The objects are placedbetween two heatable belts of the heatable fabric which run in anarcuate path and thereby, or by additional means, hold the objects underpressure. The sheathed conductors form the weft in the heating fabric ofthe belts and are connected while they pass over the first part of thearc; they are switched off when they run over the second part. Theobjects are therefore kept under pressure constantly, both during theheating and during the cooling period without a pressure release at thetransition. This double web heating and cooling under pressure issuitable for welding of plastic films to foils, for laminating sheets,films, foils, etc., Connection can be effected by bushes or fixedconductors arranged on the supporting frame, with which connections tothe limbs of the conductors make contact at the appropriate times.

FIG. 7 illustrates an example of two conveyors used to heat objectsunder pressure.

Two conveyors 35 and 36 are essentially heating fabrics as abovedescribed. They are superposed and driven by rollers 37, 38. The innerconveyor 35 is guided over an arcuate support 39. The outer conveyor isguided over guide rollers 41, 42, 43, 44. The return runs of the twoconveyors move at the same speed; the flexibility of the fabrics allowsfor the small differences in the operative runs due to the differentlengths they occupy. The roller 41 guides the conveyor 36 so far fromthe conveyor 35 that the objects to be heated and pressed can be pushedin between them from the side (either manually or automatically). Therollers 42, 43 are positioned so that the tangent to both while clearingthe objects at the location of the rollers is closer to the conveyor 35than the height of the objects. Thus immediately after the objects passthe roller 42 the conveyor 36 presses on them. The roller 44 guides theconveyor 36 in a path which is clear of the objects so that they caneasily be removed to the side.

Both conveyors are energised over part of the path from the roller pair41, 42 to the roller pair 43, 44. Over this part of the path heat issupplied to them. Over the remainder they cool down and this may beassisted by an air current. Energisation can be by means of fixedcontact rails one of which is marked 44. Against this spring brushesindicated at 45 on the conveyor, connected each to a limb or group oflimbs of the conveyor 35 make contact with the rails over acorresponding part of the path of the conveyor. Similar provision, rail46, brushes 47, is made for the conveyor 36. The rails and brushes mustbe at least in duplicate, suitably on opposite sides, to enable acomplete circuit to be made through the respective conductors of theconveyors.

What I claim is:

1. An electric heating device comprising a plurality of flexible limbseach constituted by a generally flat section electric conductor forminga heating element and a sheath of electrically non-conductive materialloosely enclosing the conductor, the adjacent limbs being continuouswith one another and disposed in a zig-zag formation with the limbsspaced apart and disposed with their flat surfaces substantiallyparallel so that when energized all radiate heat in the same generaldirection, terminal means at least at the ends of the zigzag formationwhereby the heating elements constituted by the limbs can be connectedto a supply of electrical energy, and means of electricallynon-conductive material supporting the zigzag formation at the apicesthereof with negligible friction and exerting longitudinal tensionsolely on the sheaths of the limbs whereby the sheaths of the limbs areheld suspended in the said disposition all under substantially the samelongitudinal tension.

2. An electric heating device according to claim I having the limbsspaced apart in a direction generally perpendicular to the length and inthe direction of their widths whereby the atmosphere in which the heateris used can flow over and between them and be heated by convection.

3. An electric heating device according to claim 2 in which the spacingapart of said limbs is adjustable thereby enabling the ratio of heatdissipated by radiation to heat dissipated by convection to be varied.

4. An electric heating device according to claim 2 in which the spacingapart of said limbs differs in different parts of the area occupied bythe device.

5. An electric heating device according to claim 1 in which the apicesof the zigzag formation are supported by insulating members of circularcross section freely rotatable on supporting means.

6. An electric heating device according to claim 1 in which the two setsof apices of the zigzag formation are in a respective straight line,each apex being supported by an insulator of cylindrical form, theinsulators at each set of apices being freely rotatable on a respectivesup port, at least one of the supports being under a constant forcetending to draw it away from the other support.

'7. An electric heating device according to claim 6 in which therespective supports are mounted in an external skeleton framework onesupport being fixed in the framework and the other being attached to theframework by a plurality of tension springs disposed at intervalsbetween the limbs.

8. An electric heating device according to claim 1, in which spacedflexible supporting strips of electrically nonconducting material areinterwoven with the limbs.

9. An electric heating device according to claim 8 in which thesupporting strips are held under longitudinal tension.

10. An electric heating device according to claim 1 in which the sheathsare of insulating character and adequate to space the conductors fromone another and from any framework in which the limbs are suspended, butare insufficient of themselves to effect adequate electric insulation ofthe conductors under all conditions of operation to be expected.

11. An electric heating device according to claim 1 in which the sheathsare porous.

12. An electric heating device according to claim 1 in which the sheathsconsist each of a single layer of glass fibre fabric.

13. An electric heating device according to claim 1 in which theconductor is a crimped metal foil with the crimps running transverse tothe length.

14. An electric heating device according to claim 1 in which each limbincludes a plurality of distinct conductors each being a strip ofcrimped metallic foil with the crimps running transverse to the length.

15. An electric heating device according to claim 1 also including acover on one side of the area occupied by at least some of the limbs,disposed to reflect back radiated heat so that the greater part of thetotal radiation will be in a direction away from the cover.

16. An electric heating device according to claim 1 in which at leasttwo sets of limbs are superposed spaced apart.

References Cited UNITED STATES PATENTS 1,065,015 6/1913 Youmans 338-280X 2,462,607 2/1949 Browne 338-316 X 2,482,050 9/1949 Yejeski 219-545 X2,665,356 1/1954 Du Bois 338-316 X 2,817,737 12/1957 Morris 338-208(Other references on following page) References Cited UNITED STATESPATENTS Duffy et a1. 13-22 Balestrini 338-58 Shoup 991 Eisler 174117Lund et a1. 219529 Murphy 219-528 Cox 338212 Burdge et a1. 338316 10FOREIGN PATENTS 369,225 6/1963 Switzerland.

VOLODYMYR Y. MAYEWSKY, Primary Examiner US. Cl. X.R.

